1. Field of the Invention
The present invention relates to a shock absorber, and more particularly relates to a shock absorber which is ideal as a shock absorbing pad incorporated in a door trim, center pillar trim, and the like, incorporated inside a vehicle, for absorbing a shock received by a passenger when the vehicle suffers a collision.
2. Description of the Related Art
To absorb a shock which a passenger receives in the upper torso during a vehicular collision, a shock absorbing material must exhibit stress which is linear with respect to the compression stroke (compressive distortion). That is, the material should have an F-S waveform (force-stroke waveform) wherein the stress is proportional to the compression stroke. Conventionally, an iron plate, semi-rigid polyurethane foam, and the like have been used in many cases as materials having this sort of a linear F-S waveform.
However, although an iron plate, one of the conventionally used shock absorbing materials, has a linear F-S waveform as shown in FIG. 2A, it has a drawback in that it is heavy and consequently increases the weight of the vehicle.
On the other hand, semi-rigid polyurethane foam has the F-S waveform shown in FIG. 2B, which has a drawback in that the F-S waveform is linear only during the initial period of compression (when the compression stroke is small), and deviates from the required waveform when the compression stroke increases.
Rigid polyurethane foam is generally used as a buffer material, but even a rigid polyurethane foam with a low level of hardness, having a compressive stress of approximately 3.0 kg/cm.sup.2, or a rigid polyurethane foam with a high level of hardness, having a compressive stress of approximately 8.0 kg/cm.sup.2, has a waveform which deviates considerably from the required waveform, as shown schematically by the F-S waveform of FIG. 2C.
As shown in FIG. 8 (vertical cross-sectional view), shock energy absorbers 82, for protecting the chest to shoulder part of the passenger in a vehicular collision, are provided at necessary positions on the rear face of a door trim 81 of a vehicle (the face on the opposite side to the inside of the vehicle). The energy absorbers 82 usually consist of a material with excellent energy-absorbing characteristics such as rigid polyurethane foam, or bead-like foam bodies of polypropylene, polyethylene, or a polyolefine-type resin, and are securely attached to the door trim 81 using hot-melting adhesive or double-sided tape or the like.
Each of the above materials of the energy absorbers has a comparatively high level of hardness, and therefore the energy absorbers 82 have low shock-durability and tend to break during a collision. Consequently, when the conventional energy absorbers 82 are used, as for instance in the case shown in FIG. 8, where an MDB (Moving Deformable Barrier) 85 has collided sideways from the outer plate 83 side, the shape of the outer plate 83 becomes distorted and the MDB 85 enters the inside of the vehicle, and as a result, the energy absorbers 82 are broken by the shock and are shattered. Then, after the energy absorbers 82 have been lost, the crash dummy 84 collides with the door trim 81, and as a result, no energy-absorbing effect is obtained from the energy absorbers 82.